1. Field of the Invention
This invention relates generally to the purification of water and, in particular, to the removal of dissolved solids from produced water formed from natural gas wells associated with bituminous coal deposits.
2. Description of Related Art
Pollution is a serious environmental problem for both industry and the public. Of particular concern are streams and lakes used as water supplies and government regulations limit the amount of various substances which may be discharged into and/or present in the water. In general, there are a number of different methods for the purification of water which include mechanical treatment by sedimentation, filtration or membranes; chemical purification by use of chemicals, gases or resins; and biological treatment by mixing water with bacteria to convert pollutants to innocuous byproducts.
Waste water from industrial processes typically contains a wide variety of undesirable components which restrict its use or safe disposal into lakes and streams. Standards regulating the content of waste water disposed into natural bodies of water or injected underground have become more and more strict over the years and is of particular environmental concern. One industrial waste water which is produced in large quantities is termed produced water and is generated in numerous processes including the operation of natural gas wells. Produced water typically contains large amounts of dissolved solids and the term total dissolved solids (TDS) represents the amount of total dissolved solids in the water stream.
Produced water contains a wide variety of components depending on its origin and for convenience the following description will be specifically directed to produced water formed in the operation of natural gas wells and in particular those associated with bituminous coal deposits. It will be appreciated by those skilled in the art however that the method and apparatus of the invention may be used to treat other waste waters including produced water to reduce the TDS of the water for safe discharge into lakes and streams.
In the operation of natural gas wells especially those associated with bituminous coal deposits, natural gas produced from the deposits is associated with high TDS ground water. The ground water is entrained with the natural gas as it comes to the surface and the ground water becomes a serious disposal problem since the TDS of the produced water is too high to be discharged to the surface streams surrounding the gas fields.
The produced water from these type fields typically contain greater than about 1000 ppm TDS with the large majority of the dissolved solids being in the form of sodium bicarbonate. Smaller amounts of calcium, magnesium, potassium and barium carbonates are also typically present. To safely discharge such produced water into surface streams it is usually necessary to reduce the TDS to less than 100 ppm and preferably less than 50 ppm, e.g., 25 ppm typically. It is important that the method and apparatus used to lower the TDS of the waste stream be efficient and cost effective.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for treating produced water for lowering the total dissolved solids of the water for safe environmental discharge to surface streams and other water supplies.
It is another object of the present invention to provide an apparatus for treating produced water for lowering the total dissolved solids of the water for safe environmental discharge to surface streams and other water supplies.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
In accordance with the above goals and objectives, in the present invention there is provided in one aspect a method for treating produced water to lower the total dissolved solids (TDS) of the water for safe environmental discharge comprising the steps of:
passing the produced water through a cation exchange resin, preferably a weak acid cation resin in a H+ form, to remove cations (dissolved solids) and form a reduced dissolved solids produced water stream and a solids loaded resin;
passing the reduced dissolved solids produced water stream, preferably with air, through a decarbonator (mixer) apparatus to remove formed gases such as CO2 as an off gas stream and forming a first discharge stream having a reduced TDS concentration;
discharging the first discharge stream to the environment;
regenerating the solids loaded resin to a H+ form with a dilute acid solution, preferably a H2SO4 solution, and preferably diluted with produced water, to remove solids from the solids loaded resin into the dilute acid (sulfuric acid) stream forming an acid regenerated waste stream and an acid regenerated resin;
rejecting the acid regenerated waste stream to a deep well or environmentally safe reject site;
rinsing the acid regenerated resin with water, preferably produced water, to remove residual salts from the acid regenerated resin forming a residual salt containing rinse stream;
passing the residual salt containing rinse stream into a separation unit to remove salt from the stream and preferably a reverse osmosis unit forming a reverse osmosis reject stream and a reverse osmosis product discharge water stream;
rejecting the reverse osmosis reject stream (high salt) to a deep well or other environmentally safe reject site; and
discharging the reverse osmosis product water discharge stream (low salt) to the environment.
In another aspect a method for treating produced water to lower the total dissolved solids (TDS) of the water for safe environmental discharge is provided comprising the steps of:
passing the produced water through a cation exchange resin, preferably a weak acid cation resin in a H+ form, to remove cations (dissolved solids) and form a first discharge stream and a solids loaded resin;
discharging the first discharge stream to the environment;
regenerating the solids loaded resin to a H+ form with a dilute acid solution, preferably a H2SO4 solution, and preferably diluted with produced water, to remove solids from the solids loaded resin into the dilute acid (sulfuric acid) stream forming an acid regenerated waste stream and an acid regenerated resin;
rejecting the acid regenerated waste stream to a deep well or environmentally safe reject site;
rinsing the acid regenerated resin with water, preferably produced water, to remove residual salts from the acid regenerated resin forming a residual salt containing rinse stream;
passing the residual salt containing rinse stream into a separation unit to remove salt from the stream and preferably a reverse osmosis unit forming a reverse osmosis reject stream and a reverse osmosis product discharge water stream;
rejecting the reverse osmosis reject stream (high salt) to a deep well or other environmentally safe reject site; and
discharging the reverse osmosis product water discharge stream (low salt) to the environment.
In a further aspect of the invention, the treated water of the above methods for discharge to the environment may be treated with lime or other such materials to control the sodium/calcium ratio, pH and the like of the discharge water.
In another aspect of the invention an apparatus is provided for treating produced water to lower the total dissolved solids of the water for safe environmental discharge comprising:
a resin, preferably a weak acid cation resin and preferably in a H+ form, held in a tank or other container, for contacting produced water to remove cations and form a reduced dissolved solids produced water stream and a solids loaded resin;
mixing means to mix the reduced dissolved solids produced water stream with air to remove formed gases such as CO2 as an off gas stream and form a first discharge stream having reduced total dissolved solids, which first discharge stream is discharged to the environment;
means for adding an acid, preferably a weak acid, such as a dilute sulfuric acid stream (and preferably diluted with produced water) to the solids loaded resin forming an acid regenerated resin and an acid regenerated waste stream which stream is discharged to a deep well or other environmentally safe reject site;
means for adding rinse water, preferably produced water being treated, to the acid regenerated resin to form a residual salt containing rinse stream; and
a separation unit to remove salt from the stream and preferably a reverse osmosis unit to treat the residual salt containing rinse stream to form a reverse osmosis reject stream (high salt) for discharge to a deep well or other environmentally safe reject site and a reverse osmosis product water discharge stream (low salt) for discharge to the environment.
In another aspect of the invention an apparatus is provided for treating produced water to lower the total dissolved solids of the water for safe environmental discharge comprising:
a resin, preferably a weak acid cation resin and preferably in a H+ form, held in a tank or other container, for contacting produced water to remove cations and form a first discharge stream for discharge to the environment and a solids loaded resin;
means for adding an acid, preferably a weak acid, such as a dilute sulfuric acid stream (and preferably diluted with produced water) to the solids loaded resin forming an acid regenerated resin and an acid regenerated waste stream which stream is discharged to a deep well or other environmentally safe reject site;
means for adding rinse water, preferably produced water being treated, to the acid regenerated resin to form a residual salt containing rinse stream; and
a separation unit to remove salt from the stream and preferably a reverse osmosis unit to treat the residual salt containing rinse stream to form a reverse osmosis reject stream (high salt) for discharge to a deep well or other environmentally safe reject site and a reverse osmosis product water discharge stream (low salt) for discharge to the environment.